1. Field of the Invention
This invention relates to control systems for electric machines, an encoder for an electric machine from which the position of the rotor relative to the stator is derivable and methods of starting electric machines. The invention is particularly, though not exclusively, applicable to switched reluctance (SR) machines.
2. Description of Related Art
The switched reluctance drive is a variable-speed drive system comprising an electric motor supplied from a power-electronic converter under the control of low-power control electronics. The motor has salient poles on both the stator and the rotor, typically with an excitation coil around each stator pole. These stator coils are grouped to form one or more phase windings. The electrical currents in the windings are typically switched on and off by power-electronic switches.
The timing of the switching of the currents in the windings is controlled in relation to the relative angular position of the stator and rotor poles. This relative position may be detected by a rotor position transducer which consists of a rotating member and stationary sensors which supply signals to the control electronics. In single and two-phase systems only a single sensor is required, but motors with more than two phases generally use more than one sensor according to the prior art.
FIG. 1 shows a schematic diagram of a typical 3-phase SR motor, which has six stator poles and four rotor poles. The poles carrying coils A and A' have opposite magnetic polarisation. Phases B and C are formed similarly.
A rotor position transducer (RPT) is used to ensure that the currents of the phase windings are switched on and off at the appropriate angles of rotation. FIG. 2a illustrates a typical system consisting of a rotating slotted disc and three optical sensors which are switched by the rotation of the disc.
FIG. 2b shows the idealised variation of the inductances of the phase windings as functions of angle of rotation .theta.. Positive torque is defined as that which acts in the direction to more the rotor in the direction of positively increasing .theta.. Such torque is produced by any phase when the winding of that phase carries current and the angle of rotation is such that the inductance is increasing with increasing .theta.. Negative torque is produced when a phase carries current during that part of its inductance cycle where the inductance is decreasing with increasing .theta. (or increasing with decreasing .theta.).
It will be apparent therefore that for normal positive torque operation in the direction of increasing .theta. (the forward direction) each phase is energised in turn when its inductance is increasing. FIG. 2b also shows the three output signals of the sensors according to prior art. In general the RPT consists of three sensors, which may for example, be optical, magnetic or inductive, which cooperate with a rotating member, for example a disc with cut out slits, to produce signals such as those shown in FIG. 2b as RPT.sub.A, RPT.sub.B and RPT.sub.C. Thus the signal RPT.sub.A changes from zero to a positive value at the centre of the minimum inductance region of phase A and returns to zero at the maximum inductance position of phase A. The signals RPT.sub.B and RPT.sub.C behave in the same manner for phases B and C respectively. It will be clear that whether the rotor is starting from rest in either the forward or backward direction or running in either direction, the RPT signals enable the control electronics to excite the appropriate phase winding over the appropriate angle of rotation to produce torque in the desired direction.
The torque developed by the motor may be controlled at low speeds by adjusting the magnitude of the current in the phase windings over the fixed angle defined by the respective positive or negative RPT signals depending on the desired direction of torque. At high speed the torque is normally controlled by adjusting the angle over which a phase winding is switched onto the power supply and the angle with respect to the inductance cycle at which the switching on takes place, i.e. by timing the switching on and off of the phase energisation with reference to the RPT signals. It will be clear from FIG. 2b that the combined RPT signals can be used to determine the rotor position to one sixth of a phase period thus giving a 15 degree resolution. For more refined control of switching angles, each of these six regions can be interpolated, for example, by using a high frequency pulse train which is phase locked to the low frequency RPT signals as described in GB 1597790 (Stephenson).
U.S. Pat. No. 4,990,843 (Moren) describes a method by which the three detectors of FIG. 2a may be replaced by a single detector. The rotating disc incorporates six or twelve slots, giving a sensor signal which has respectively a rising or falling edge every 30 degrees of rotation, or a falling edge every 30 degrees of rotation, which may be used to switch the phase winding currents of the motor. However, the method suffers from the serious disadvantage that correct tracking of the relationship between edges and phases may be lost due to electrical noise, for example, in the sensor signal.